Lifetime breast cancer risks among carriers of mutations of the genes BRECA1 and BRCA2 have been estimated at 40-80%. The fact that some 20-60% of carriers live to advanced ages without developing the disease suggests that other genes or environmental exposures may modify carriers' breast cancer risks. At present however, little is known about personal and environmental characteristics that modify these risks. For example, little is known about how a carrier's risk varies with her reproductive and menstrual history, her diet and alcohol intake, her physical activity patterns, and her exogenous hormonal exposures. Many of these attributes are well-established risk factors for breast cancer in the general population of non-carriers. There is urgent need to determine which, if any, modifiable lifestyle characteristics may alter a carrier's risk of developing breast cancer, to assist her in making rational, informed choices about such preventive options as prophylactic mastectomy and the use of chemopreventive agents such as tamoxifen and raloxifene. We propose to use the CFRBCS data to analyze the relation between breast cancer risk and specific environmental and hormonal attributes among an anticipated 800 female carriers of mutations of BRCA1 or BRCA2. Attributes include: 1) reproductive characteristics; 2) exogenous hormone use; 3) diet; 4) body size; 5) physical activity; 6) radiation exposure; 7) alcohol consumption; and 8) cigarette smoking. We will use a breast cancer to those of all female carriers in her birth cohort who were free of breast cancer affected with breast cancer to those of all female carriers in her birth cohort who were free of breast cancer at the age when she was diagnosed. This large group of more than 800 carriers of BRCA1 or BRCA2 mutations represents a unique resource for advancing our knowledge about breast cancer management in women at high risk for the disease.